A position of a device can be determined by means of various positioning methods. Some of these methods may profit from the availability of assistance data.
A positioning of a device is supported for instance by various Global Navigation Satellite Systems (GNSS). These include for example the American Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS), the future European system Galileo, the Space Based Augmentation Systems (SBAS), the Japanese GPS augmentation Quasi-Zenith Satellite System (QZSS), the Locals Area Augmentation Systems (LAAS), and hybrid systems.
The constellation in GPS, for example, consists of more than 20 satellites that orbit the earth. Each of the satellites transmits two carrier signals L1 and L2. One of these carrier signals L1 is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier phase is modulated by each satellite with a different C/A (Coarse Acquisition) code. Thus, different channels are obtained for the transmission by the different satellites. The C/A code is a pseudo random noise (PRN) code, which is spreading the spectrum over a 1 MHz bandwidth. It is repeated every 1023 bits, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with navigation information at a bit rate of 50 bit/s. The navigation information comprises inter alia ephemeris and almanac parameters. Ephemeris parameters describe short sections of the orbit of the respective satellite. Based on these ephemeris parameters, an algorithm can estimate the position of the satellite for any time while the satellite is in the respective described section. The almanac parameters are similar, but coarser orbit parameters, which are valid for a longer time than the ephemeris parameters. The navigation information further comprises for example clock models that relate the satellite time to the system time of GPS and the system time to the Coordinated Universal Time (UTC).
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and it detects and tracks the channels used by different satellites based on the different comprised C/A codes. Then, the receiver determines the time of transmission of the code transmitted by each satellite, usually based on data in the decoded navigation messages and on counts of epochs and chips of the C/A codes. The time of transmission and the measured time of arrival of a signal at the receiver allow determining the pseudorange between the satellite and the receiver. The term pseudorange denotes the geometric distance between the satellite and the receiver, which distance is biased by unknown satellite and receiver offsets from the GPS system time. Moreover, pseudorange contains various error terms including troposphere and ionosphere delay as well as multipath.
In one possible solution scheme, the offset between the satellite and system clocks is assumed known and the problem reduces to solving a non-linear set of equations of four unknowns (3 receiver position coordinates and the offset between the receiver and GPS system clocks). Therefore, at least 4 measurements are required in order to be able to solve the set of equations. The outcome of the process is the receiver position.
Similarly, it is the general idea of GNSS positioning to receive satellite signals at a receiver which is to be positioned, to measure the pseudorange between the receiver and the respective satellite and further the current position of the receiver, making use in addition of estimated positions of the satellites. Usually, a PRN signal which has been used for modulating a carrier signal is evaluated for positioning, as described above for GPS.
In some environments, a GNSS receiver may be able to acquire and track sufficient satellite signals for a positioning based on the PRN codes, but the quality of the signals may not be sufficiently high for decoding the navigation messages. This may be the case, for instance, in indoor environments. Further, the decoding of navigation messages requires a significant amount of processing power, which may be limited in a mobile GNSS receiver.
Moreover, a satellite signal is distorted on its way from a satellite to a receiver due to, for instance, multipath propagation and due to influences by ionosphere and troposphere. While the GNSS satellites may transmit ionosphere models for correcting the signals accordingly, the provided models may be not very accurate or up-to-date.
If the GNSS receiver is included in a wireless communication terminal or attached as an accessory device to a wireless communication terminal, a wireless access network may therefore be able to provide the wireless communication terminal via a radio link with assistance data. Assistance data is typically provided for each satellite that is visible to the GNSS receiver associated to the cellular terminal. The assistance data may comprise navigation model parameters, which usually include orbit parameters, time of ephemeric (TOE) and time of clock (TOC) parameters and satellite identity parameters. Further, correction data which take account, for example, of GNSS signal delays caused by the atmosphere and the ionosphere, may be provided as assistance data. Such a supported GNSS based positioning is referred to as assisted GNSS (AGNSS). The received information enables the GNSS receiver or the associated wireless communication terminal to obtain a position fix in a shorter time, in more challenging signal conditions or more accurately.
Assistance data for assisted navigation systems, such as GNSSs like GPS, GLONASS and Galileo, have been specified and standardized for various cellular systems. The delivery of such assistance data can be built on top of cellular system-specific control plane protocols including, for example, the radio resource location services protocol (RRLP) for the Global System for Mobile Communications (GSM) networks, the radio resource control (RRC) layer of layer 3 in wideband code division multiple access (WCDMA) networks, and IS-801 for CDMA networks.
The Open Mobile Alliance (OMA) has moreover defined a user plane protocol referred to as secure user plane location (SUPL). SUPL employs user plane data bearers for transferring location assistance information such as GNSS assistance data and for carrying positioning technology-related protocols between terminal, for example a mobile communication device, and its operating network. SUPL is intended to be an alternative and, at the same time, a complement to the existing standards based on signaling in the mobile network control plane. SUPL assumes that a mobile or other network can establish a data bearer connection between a terminal and some type of location server. The use of a user plane protocol becomes especially appealing in case of internet protocol (IP) networks where the data bearer is available by nature.